Due to a slight change in the RPI B+ rev1.2 (Hynix memory) my old image won't boot on a new RPI B+, so I decided to restart with a jessie image.
On the previous version the UART (coming out on pins 8 and 10 at the RPI header) worked very well, but on this one (Linux version 4.4.21+ with Jessie image) I cannot get it to work.

I have added the following three lines, by themselves or in combinations to the /boot/config.txt:
enable_uart=1
dtparam=uart1=on
dtoverlay=uart1,txd1_pin=14,rxd1_pin=15

But after rebooting the pins still stay assigned as normal IO:[email protected]:~# raspi-gpio get 14
GPIO 14: level=1 fsel=0 func=INPUT

What would it take to connect the UART?

I have some other IO's as well, maybe it's best to make an overlay for the application?
Where can I find explanation how to do this?
Or is there a way to go back to the old style, because these RPI's will only be used for this application anyway?

* uart0 is a pl011, also known as /dev/ttyAMA0.
* uart1 is an 8250 clone, also known as the mini-UART and /dev/ttyS0.

UART0 is the natural UART to use on any Pi without on-board Bluetooth - with no config.txt settings it will be enabled by default (enable_uart=1 unless you change it).
UART0 is (by default) used for Bluetooth on Pi 3 and Pi Zero W, so UART1 is the free UART. UART1 has smaller FIFOs, dodgy flow control and has a clock derived from the core/VPU clock, so making use of it requires fixing the VPU clock to 250MHz. enable_uart=0 by default on these Pis - changing it to 1 will also lock the core to 250MHz.

You can't use both UARTs on a non-BT Pi other than a Compute Module, so do yourself a favour and remove those config.txt settings (you can leave enable_uart=1, although it is unnecessary) and use /dev/serial0 (an alias to the primary, console UART - UART0/ttyAMA0 on non-BT Pis, UART1/ttyS0 on BT Pis).

And on the subject of doing yourself a favour, why would you choose Jessie when starting a new image? Our Stretch releases are better in many ways.

I am using jessie because have another application that was written already for the 'hynix' version a while ago, which uses a lot of the same libraries and settings as the one I am trying to get working now.
That is why I am trying to start from that image.

I want to use the pl011 uart (/dev/ttyAMA0), which is the one I was using before.
How do I tell the PI B+ to connect that to pins 8 and 10, instead of standard IO?

Or is there a way to make the old image boot on a RPI 1 B+ V1.2 with the Hynix memory chip?
A lot of time has gone in to configuring, developing and testing this application, so I would like to change as little as possible, to avoid introducing new bugs.

The firmware and kernel packages shipped with Jessie should enable UART0 automatically on a B+ unless there is something in your config.txt preventing it. It might be worth uploading config.txt and the output from "dmesg" after booting.

# uncomment if you get no picture on HDMI for a default "safe" mode
#hdmi_safe=1

# uncomment this if your display has a black border of unused pixels visible
# and your display can output without overscan
#disable_overscan=1

# uncomment the following to adjust overscan. Use positive numbers if console
# goes off screen, and negative if there is too much border
#overscan_left=16
#overscan_right=16
#overscan_top=16
#overscan_bottom=16

# uncomment to force a console size. By default it will be display's size minus
# overscan.
#framebuffer_width=1280
#framebuffer_height=720

# uncomment if hdmi display is not detected and composite is being output
#hdmi_force_hotplug=1